This invention relates to an engine for a supersonic aircraft and more particularly to an inlet and fan combination resulting in a significantly shorter, lighter and more efficient overall propulsion system for supersonic aircraft. A design method is also disclosed.
Air entering an engine of a supersonic aircraft must be slowed down from a supersonic speed to a subsonic speed in order for the engine to perform properly. A typical engine for use with a supersonic aircraft is shown in FIG. 1. A rather long inlet section 10 (typically exceeding the length of the engine itself) of a prior art engine 12 acts as an interface between external freestream air and the engine. The deceleration is achieved by varying the flow area and through a series of shock waves that develop in the inlet section 10. In this conventional inlet more than half the length (and weight) is typically required to decelerate the flow to subsonic conditions suitable for engine entry. For example, if the external freestream air velocity is Mach 2, the inlet section 10 will decelerate the velocity to approximately Mach 0.5 at the face of a fan 14. The largest contribution to the pressure loss also accumulates during this deceleration process in the inlet section 10.
In conventional turbofan engines such as the engine 12 of FIG. 1 the front fan 14 provides some fraction of the propulsive thrust and is driven by the core engine. The fan raises the total, or stagnation, pressure of the incoming air by adding work to the flow. A fraction of the air goes into the core engine and the remaining fraction (bypass air) is expelled through a nozzle for propulsive thrust. Conventional fans and engines are designed for a subsonic entry flow and subsonic exit flow.
Another known art engine for supersonic aircraft is the supersonic through-flow fan (STF) engine developed by NASA for the supersonic transport program. This engine is shown in FIG. 2. The fan 14 of this STF engine is based on an impulse blade design that adds work to the flow exclusively by turning the flow through a large angle in the relative frame of reference. There is a static pressure drop in the flow and the flow is accelerated to a higher velocity at the fan exit. The shock system in this fan includes weak oblique shocks and expansion fans, and the boundary layer remains well attached as there is a mild adverse pressure gradient on the blade surface. It is important to note that this STF fan design has absolute frame supersonic inlet and exit flow and in fact accelerates the flow to a higher Mach number resulting in performance penalties and implementation challenges. Although the length of the inlet 10 upstream of the fan 14 of the engine in FIG. 2 is reduced, this reduced length is merely traded for a much more complex problem of designing an efficient, light diffuser after the flow leaves the fan. (See, Schmidt, J. F., Moore, R. D., Wood, J. R. and Steinke, R. J., “Supersonic Through-Flow Fan Design” NASA TM-88908, 1987 and Tavares, T. S. “A Supersonic Fan Equipped Variable Cycle Engine for a Mach 2.7 Supersonic Transport,” Masters Thesis, M.I.T., August 1985.)